In many medical procedures, probes, such as endoscopes and catheters, are temporarily inserted into a patient's body. There are also procedures in which devices, such as implants, are inserted into the body permanently or for extended periods. Various methods of determining the location of these inserted medical devices are known in the art. X-ray imaging is the most commonly used location confirmation system. Position sensing systems can also be used for this purpose, and are preferable in particular when the location of the device must be tracked over an extended period.
Ultrasound intrabody position sensing is well known. Such systems commonly require an active transducer in the device that is inserted into the body, connected by wires to a console outside the body. The transducer either receives ultrasonic waves from emitters outside the body or radiates ultrasonic waves to receivers outside the body. Other ultrasonic systems use a passive ultrasound reflector in the inserted device which gives a strong reflection of ultrasonic waves irradiating the body without the necessity of running wires through the catheter. These passive systems necessarily create a strong background of ultrasonic radiation against which the position of the reflector must be found.
Other position sensing systems use electromagnetic fields. For example, PCT Patent Publication WO 96/05768 to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes a locating system for determining the location and orientation of an invasive medical instrument whereby an externally applied RF field induces a current in three coils located within the invasive medical instrument. Wires or some other form of physical leads are required to carry this induced signal from the catheter to a signal processor in the extrabody space. The processor analyzes the signal so as to calculate the location and orientation of the invasive medical instrument.
In many applications, it is advantageous to fix a wireless passive emitter, or “tag,” to the device that is inserted into the body. Such a tag contains no internal power source, but is rather actuated by an external energy field, typically applied from outside the body. The tag then emits ultrasonic or electromagnetic energy, which is detected by antennas or other sensors outside the body. The detected signals are generally used to simply to ascertain the presence of the tag within a given region (such as the abdominal cavity), although some tags may also be used to determine position coordinates. Passive ultrasonic reflectors, mentioned above, are one simple example of such tags. Other passive tags receive and re-emit electromagnetic radiation, typically with a frequency and/or phase shift. Hybrid tags, combining ultrasonic and electromagnetic interactions, are also known in the art.
For example, U.S. Pat. No. 6,026,818 to Blair et al., whose disclosure is incorporated herein by reference, describes a method and device for the detection of unwanted objects in surgical sites, based on a medically inert detection tag which is affixed to objects such as medical sponges or other items used in body cavities during surgery. The detection tag contains a single signal emitter, such as a miniature ferrite rod and coil and capacitor element embedded therein. Alternatively, the tag includes a flexible thread composed of a single loop wire and capacitor element. A detection device is utilized to locate the tag by pulsed emission of a wide-band transmission signal. The tag resonates with a radiated signal, in response to the wide-band transmission, at its own single non-predetermined frequency, within the wide-band range. The return signals build up in intensity at a single (though not predefined) detectable frequency over ambient noise, so as to provide recognizable detection signals.
U.S. Pat. No. 5,057,095 to Fabian, whose disclosure is incorporated herein by reference, describes apparatus for detecting a surgical implement in human or animal tissue, comprising a detector responsive to the presence, within an interrogation zone, of a surgical implement to which a marker is secured. The marker is adapted to produce identifying signal characteristics within a frequency band generated in the interrogation zone. Variations in the phase and or direction of the interrogating field and changes in the electromagnetic coupling between markers and receiver are intended to optimize coupling therebetween.
U.S. Pat. No. 6,076,007 to England et al., whose disclosure is incorporated herein by reference, describes a method for determining the position and orientation of a surgical device within a human body. In one application, a catheter or prosthesis is characterized in that it carries, at a predetermined location, a tag formed of a high permeability, low coercivity magnetic material. The position of the tag (and hence of the surgical device) is sensed by remotely detecting its magnetic response to an interrogating signal.
U.S. Pat. No. 5,325,873 to Hirschi et al., whose disclosure is incorporated herein by reference, describes a system to verify the location of a tube or other object inserted into the body. It incorporates a resonant electrical circuit attached to the object which resonates upon stimulation by a hand-held RF transmitter/receiver external to the body. The electromagnetic field generated due to resonance of the circuit is detected by the hand-held device, which subsequently turns on a series of LEDs to indicate to the user the direction to the target. An additional visual display indicates when the transmitter/receiver is directly above the object.
In a non-medical context, U.S. Pat. No. 3,713,133 to Nathans et al., whose disclosure is incorporated herein by reference, describes a theft-prevention system in which a piezoelectric crystal having a resonant frequency is incorporated into a device which is then attached to individual items within a store. When a radio frequency (RF) signal having a frequency equal to the resonant frequency of the crystal strikes the crystal, an oscillating electrical field gradient is produced across the face of the crystal at the radiated RF frequency, and two tin foil members mounted on the crystal vibrate, emitting ultrasound. Detection of the ultrasound under appropriate conditions produces an alarm, indicative of an attempt to remove the item from the store without authorization. In another embodiment, a small, thin metal diaphragm vibrates when irradiated with an ultrasound field at or near the resonant frequency. The vibration of the diaphragm induced by the ultrasound field modulates an incident RF field, and the modulation is detected by an RF transducer to activate the alarm. These systems do not provide specific information describing the location of the item, but only that the item has entered a detection area (typically near an exit from the store).
Passive sensors and transponders, fixed to implanted devices, can also be used for conveying other diagnostic information to receivers outside the body. For example, U.S. Pat. No. 6,053,873 to Govari et al., whose disclosure is incorporated herein by reference, describes a stent adapted for measuring a fluid flow in the body of a subject. The stent contains a coil, which receives energy from an electromagnetic field irradiating the body so as to power a transmitter for transmitting a pressure-dependent signal to a receiver outside the body. In one embodiment, the transmitter is based on a tunnel diode oscillator circuit, suitably biased so as to operate in a negative resistance regime, as is known in the art.
As another example, U.S. Pat. No. 6,206,835 to Spillman et al., whose disclosure is incorporated herein by reference, describes an implant device that includes an integral, electrically-passive sensing circuit, communicating with an external interrogation circuit. The sensing circuit includes an inductive element and has a frequency-dependent variable impedance loading effect on the interrogation circuit, varying in relation to the sensed parameter.